Carrier phase-related systems include systems that compute useful information by receiving a signal at two different locations. The signal generally includes data encoded on a carrier. In certain applications, the phase of the carrier of the received signal is measured at each location and the difference in phase is used to determine positioning characteristics. Carrier phase-related systems include applications in kinematic positioning and carrier-smoothed code differential positioning. In one positioning application, carrier phase-related systems are used in conjunction with a GNSS (Global Navigational Satellite System) such as Global Positioning System (GPS)/Navstar or GLONASS for altitude determination of a vehicle.
GPS is a navigational system used by military and civilian naval, ground and airborne vehicles. GPS includes 24 Navstar satellites (SVs) deployed in 12-hour orbits about Earth and dispersed in six orbital planes at an altitude of 11,200 miles. The satellites continuously emit electronic GPS signals (or telemetry) for reception by naval, ground and airborne receiver units (FIG. 1). By receiving GPS signals from four or more satellites, a properly configured receiver unit can accurately determine its position in three dimensions and time.
The maturation of multi-antenna GPS technology for attitude determination (including heading) has motivated investigations into its use both as a viable supplement to inertial gyro sensors in the near term, and possibly as an essential complement to them in the long term. Conventional GPS navigational systems capable of attitude determination, such as system 6 illustrated in FIGS. 1 and 2, generally include at least three antennas 12A-C coupled to at least three GPS receiving units 14A-C and a processor 16. Receiving units 14A-C "track" the GPS satellites, i.e. determine carrier phase information from the satellites. Computer 10 then determines a coordinate position for the vehicle 21 upon which it is implemented based upon satellite signal code and carrier phase information received on buses 18A-C from receivers 14A-C. Additionally, processor 16 compares satellite signal code and carrier phase information to compute attitude information at an output 28 for system 6. The calculation of the attitude information from the carrier phase information on buses 18A-C is well known in the art. Briefly, attitude determination involves measuring and analyzing integrated carrier phases from tracking a plurality of satellites to obtain three-dimensional coordinate values of an unknown point with high accuracy.
With reference to FIG. 3, there is shown a conventional kinematic processing method for attitude determination operations. The kinematic processing begin with validity checks at a step 30 whereby individual carrier and phase information from satellites 2A-D (FIG. 1) is checked. Signal-to-noise ratio, loss of lock and phase error threshold are calculated based on signal parameters to ensure the quality of each incoming signal. Next, a single differencing is done at a step 32. Single differencing refers to taking the difference in the phases of the carrier waves between two GPS signals along an antenna baseline. Single differencing removes satellite clock errors, atmospheric refraction errors, some level of multipath errors and errors caused by selective availability.
The next step is selection of a reference GPS signal at a step 34. Once one signal has been selected as the reference, a double differencing is done at a step 36. In double differencing, the single differenced results are differenced again between satellites. Double differencing removes receiver clock errors, electrical path length errors, and non-synchronous receiver errors. Next, the processing mode is determined at a step 38. The processing mode may be either "initializing" or "tracking". The mode will be "initializing" if initial integer ambiguities associated with each satellite have not been resolved on enough satellites to achieve a sufficiently precise solution computation. Conversely, the mode will be "tracking" if sufficient integer ambiguities have been resolved.
Next, an attitude solution is computed at a step 40 using well-known methods. Finally, ambiguity resolution at a step 42 occurs for any satellite signals not yet initialized; and the system 6 begins again with validity checks 30.
In attitude determination, one problem well-known in the art results from signals transmitted from the GPS satellites being subjected to various kinds of obstacles including body obscuration, body multipath, external multipath and interference, all well-known in the art. These obstacles sometimes cause an interruption, either instantly or continuously, on the receiving end 14A-C of the system 6. Until the obstacles are removed, the radio waves to be transmitted from the GPS satellites cannot be received. Consequently, the amount of changes of the integrated carrier phase during this interruption cannot be known. This kind of dropping or lacking of measured data due to receiving obstacles is called the "cycle slip" or "fault" or "failure" and is regarded to be the most insidious type of error in a GPS attitude determination system. The high fidelity information derived from carrier phase measurements rely greatly on the fact that the carrier phase is continuously and flawlessly tracked without cycle slip.
Another problem well-known in the art is that of ambiguity resolution. GPS interferometric techniques involve ambiguity in the solution space for the carrier phase measurements, which must be resolved. Specifically, the carrier phase measurement repeats every wavelength of the GPS carrier frequency, which is about 19 centimeters. Thus, the actual path difference for a first and second antenna to a selected satellite is the distance corresponding to the measured phase difference plus an unknown integer number of wavelengths. As a result, a number of path-difference solutions exist within the uncertainty region, creating an ambiguity problem at every integer wavelength that must be resolved before that satellite can be used in attitude solution computations.
Several techniques have been used to detect and/or correct for cycle slip errors. For example, errors due to body obscuration have been addressed by body profile masking, either physical or through software implementation as disclosed in U.S. Pat. No. 5,185,610 to Ward, et. al. However, masking limits available satellites and fails to address the other above-mentioned sources of cycle slip errors.
Another technique is directed to a GPS surveying system that, upon detection of a cycle slip, calculates; a correction amount and adds it back into the integrated carrier phase. However, this technique is not appropriate for a moving vehicle which does not have a constant position over time. Once movement is introduced into this system, additional uncertainties and noise result making its results difficult to achieve.
Another technique is a method for generating attitude determinations including integrity checking. This integrity checking takes advantage of the known relationship among four antennas in a plane to detect a cycle clip. Thus, the method is limited to a four-antenna system, which is not always available, particularly on an airplane where space for antennas is limited. Further, this method lacks the robustness needed for a high-integrity environment.
Many techniques are known in the art for resolving ambiguities. However, these solutions are directed to resolving ambiguities in an initialization stage, usually while an aircraft is stationary. The solution-based ambiguity resolution and resolvability test of the present invention is optimal for re-resolution of ambiguities that were lost during tracking as a result of a cycle slip or a newly visible satellite.
Thus, there is a need for an improved system for and method of detecting and correcting cycle clip in a carrier phase-related system that overcomes the above-mentioned deficiencies. Further, there is a need for a cycle slip detection and correction system which is applicable to systems with less than four antennas, such as in simple, two-antenna pointing applications. Further still, there is a need for a cycle slip detection and correction system which can correct for resulting ambiguities while tracking, and has the robustness needed for high-integrity applications.